Passivhaus

When Ted and I started this endeavor in 2009, I didn't know the slightest thing about construction. I couldn't have told you how a stud wall is built, or even described with confidence what a 2x4 looks like.

My how things change! Just last night I dreamt I was quizzing a contractor about air barriers and external insulation, while harboring deep suspicions that he hadn't properly managed the vapor barrier on the 2x6 walls of the house he was rehabbing. I was gearing up to explain why polyisosyanurate was a better choice than extruded polystyrene (XPS) for above-grade rigid insulation, since the blowing agent used to produce XPS has a much higher GWP (global warming potential) than the one used for polyiso... but then I woke up.

I honestly never intended to jump into the deep end of residential construction theory! We simply wanted to build a comfortable and energy-efficient house, which led us to Passivhaus, which led us to our energy guru Marc Rosenbaum, who opened my eyes to the rough and tumble world of building science.

I have a problem with the Passivhaus people, because they are building ugly freaking boxes. The only way to get a building to last a long time is if the building is maintained, and people have to want to take care of it. People do not take care of ugly things.

Ouch! That one hurt a bit, probably because I sometimes worry that our house is an example. The exterior is nowhere near as bad as some passive houses I've seen pictures of, but it is not quite as handsome as I would have liked:

I should mention that this is the house's least flattering angle, and it looks particularly odd without the solar collector that will attach to those two white strips on the facade (probably making the house look even odder). I'll post more flattering and accurate photos below, but you can see that we have indeed built a big freaking box.

The house didn't start out like this. We got invaluable advice from our friend Camilo, an architect in New York, and he designed something a lot more stylish:

But reality intervened, and we had to stray far from his plan. Our building site is both sloped and ledgy (i.e. lined with bedrock), so a stem-wall foundation was impractical, and we needed to shorten the house from 58' to 46' to keep the lower piers from rising ludicrously high. This meant lopping off some space (which we probably won't miss at all), and it made it easier for us to reach Passivhaus numbers, but it also made the house more boxy and less "architectural."

We were very sad to have to reverse the roofline, since it drastically reduced the amount of space for solar electric panels. We therefore added the overhang to the top of the south wall, which turned Camilo's dramatic design into a stark-looking quasi-saltbox.

That said, I don't think our house is ugly enough to invite future neglect. In real life, it looks a good deal better than that first photo would suggest. You can't really see the garage in that photo, and the garage's south-facing roofline complements the house nicely:

It looks even better from the road (which of course is how most people will see it):

And the best part, frankly, is the interior. While we mercilessly chucked most of Camilo's exterior design, we consulted him regularly about the floor plan and followed his advice even when it would have been cheaper or easier to ignore it. We refused to replace his vaulted ceiling with a standard attic, which meant we had to find joists that were deep enough for Passivhaus insulation and also strong enough to bridge a 26' clear span. We also sacrificed upstairs floor space to preserve an atrium-like expanse above the dining area. I spent countless hours in SketchUp tweaking and tuning the interior, and now that the house is drywalled I can see that my time was not entirely wasted. It really is a cool space, and it doesn't feel generic or boxlike at all.

One of the many things I like about building websites is how easy it is to change things later on. Want to change some wording or add a new photo? Piece of cake! Need to change something complicated like the page layout? That's a bit more work but 100% doable — change a few settings and template files, and you've got a new layout.

Changing a building? Not so easy. You can't just edit the source files and hit "Reload." Nope, changing a building involves crowbars, debris, dumpsters, and a whole lot of work and expense. My sole consolation about building a house from scratch is that it's apparently less painful than trying to remodel, so we're making every effort to do things right the first time around.

This has meant paying constant attention to detail and making sure we're designing for longevity. The idea of doing things right is so deeply ingrained that we don't even ponder the alternative, and so we often forget how unusual it is.

For example, Ted recently shared some photos of our garage on Google+ and received unexpected praise for the generous overhangs:

I designed the garage myself, and when it came to sizing the overhangs I simply asked Eli what he recommended and went with that. It didn't occur to me not to have overhangs, or that sizing them properly was unusual enough to attract praise.

And yet it is. When people with building experience come visit our house, they're astonished by how well-built it is and how little we've compromised. The most recent was Russ, our new rep from the building supply yard. He said he's worked with lots of customers who initially intend to build a super-insulated, passive solar, [insert eco-adjective here] house. But then they see the price for all those green-building features, and they scale it back until they wind up with a house that's only marginally more "green" than a conventional house.

Russ told us what a thrill it is to see a house where this hasn't happened. Quite the opposite — our original target was an almost-passive house, and yet here we are building what is now likely to be a certified passivhaus with solar hot water and a 3.96 kW photovoltaic array. I honestly can't think of a single corner we've cut with regard to the building's performance.

We've certainly trimmed some other corners. Heck, we lopped about 500 square feet from the original floor plan. The bathrooms are small and simple, the bedrooms have ordinary (not walk-in) closets, the kitchen cabinets are stock rather than custom. But we can't bring ourselves to install anything we'll want to rip out and upgrade in a few years. Hence no formica counters, vinyl floors, or any of the other money-saving standbys.

Alas, there is a reason so few people build this way. It's really expensive! I know I've posted about this before, but it's such a big part of our building experience that I can't help repeating myself. Nearly every estimate we receive is like getting the wind knocked out of me — it seems impossible that construction could be this expensive, and yet apparently it is.

One of the hardest parts is how often I feel ashamed or that I've somehow failed. When I started this blog, I really wanted it to describe how we built an almost-passive house for some cute number like $100 or $125 a square foot. But we've sailed past $200 a square foot, and I don't yet know what the final number will be. I'm not looking forward to taking out that mortgage, nor do I relish the possible eventuality of selling the house for less than we put in.

Ted suspects that a lot of our costs are because this is a custom home. Yes, all the eco-bling is adding a lot, but not as much as the general costs of building a distinctive home on a challenging site.

Perhaps this blog will help illustrate the need for production-scale passivhaus construction. Such construction is common in Europe, and it's beginning to take off in North America. Builders of modular homes are getting in on the game as well, and perhaps very soon it will be possible to get a house like ours much more easily and cheaply than we are. I certainly hope so, because I wouldn't wish this process on anyone, and yet I definitely want more people to have this kind of house.

Due to some prior commitments, Eli pulled his crew from our job for much of November and December, and the pace of our construction slowed down accordingly. Eli already had a busy year lined up when we first called him last spring, but he couldn't resist the lure of building southern Vermont's first (almost) Passive House. He was able to pull it off thanks to a lot of schedule-shuffling, and once we were weathered in it was our turn to be shuffled.

Fortunately, I am such a sporadic blogger that I still have heaps of photos to share. And things are swinging back into high gear, so there should be more rapid and dramatic progress soon.

Let the photos commence! Our solar panels went up in November — 3.96 kW of DC goodness. We're not hooked up yet, but that will happen by the end of this month.

The plumbing work began — I suspect a lot of rough plumbing looks like something drawn by M.C. Escher, but ours is pretty over-the-top:

With Eli's team off in Guilford, we hired a pair of building pros to put up our exterior polyiso (rigid foam) insulation , housewrap, and strapping. We probably won't put up the siding until spring, which means our house will have that classy Typar look for several months. Eli once mused that they ought to sell housewrap printed to look like painted clapboards, but that most Vermonters would then never bother putting up real siding on top of it. I concur.

Anyway, here's the polyiso going up the north wall:

I am pleased to report that Ted's collarbone surgery was a success and that he is recovering nicely. This means that he is back in the DIY saddle, and for the last week we've been busy constructing the two loft spaces.

Ted at the chopsaw:

Before sharing the photos, I should explain the two loft spaces. We haven't built any of the upstairs interior walls yet, because we want to install the ceiling drywall all at once, creating a nice uninterrupted vapor-barrier. But we can't put up drywall until the roof insulation goes in, and it's hard to do any of that with a ceiling that's 25 feet high in spots (above the staircase and the two-story dining area).

Ted and I therefore decided to build the two loft platforms — one is a utility loft over the upstairs office area (it will house the PV inverter and the heat recovery ventilator), and the other is a cozy space we've dubbed the "manatee cave." Ted and I are inordinately fond of manatees (especially dwarf land manatees), and we thought that a manatee cave was much more original than a mere man-cave.

Anyway, here's the upstairs before we started building the lofts:

And here it is afterward:

The manatee cave is on the left, and in the foreground you can see the utility loft. Today we built a temporary platform connecting the two lofts (to facilitate the insulation and drywall work), and tomorrow we plan to build a platform between the utility loft and the west wall, which is where the curved staircase will go (the current staircase is merely temporary).

This last photo won't be very exciting to anyone but a building science geek, but it is very exciting to us. It's the readout from our first blower-door test, taken in December. A blower-door test quickly measures how much a house leaks. If Ted and I want to achieve Passivhaus certification, we need to build an insanely tight house (0.6 air exchanges per hour when pressurized to 50 pascals, roughly equivalent to 0.04 air exchanges per hour in a non-pressurized house).

We had no intention of running a blower-door test this early, because we haven't begun to seal the obvious leaks. The walls and roof are already very tight, but we can still see daylight at the window corners, and the exterior subfloor is completely absent (we need to finish the rough plumbing and electrical work first). But we wanted to insulate the roof so we could put up the ceiling drywall and build the interior walls, so we arranged a preliminary blower-door test to check for leaks around the roof (much harder to fix once the insulation is in). We taped over some of the gaping leak points to prevent them from wrecking the test altogether, but otherwise we haven't done any post-construction sealing.

We weren't expecting a result anywhere near the Passivhaus requirement. So we were quite happily gobsmacked to discover we're very nearly there:

The readout shows that the pressurized house is leaking at 227 CFM, which in our house converts to 0.65 ACH@50Pa. Which means that before we've even insulated or added the ZIP-sheathing subfloor, we are within spitting distance of the rigorous Passivhaus requirement. The test was run by Bill Hulstrunk of National Fiber, and he said that of the thousands of houses he's tested, this was the tightest house he'd ever seen. I am too superstitious to remove the "Almost" from the name of this website, but things are looking good in that department!

Work is now kicking back into high gear — in the near future we hope to get roof insulation, ceiling drywall, rough electrical wiring, and a garage. Our winter has been largely snow-free, a fortunate circumstance I attribute to Ted's and my purchase of a heavy-duty snowblower and season lift tickets at Stratton and Okemo. So I hope to have lots of thrilling new photos soon.

I recommend the book highly even if you don't actually wind up building what's in it, because the drawings are really helpful for understanding how to avoid thermal bridges, how to detail the airtight seals between floors, walls and ceiling, and also for ideas about what sort of material to use. I searched it carefully for details that would work for our foundation, but it didn't cover the case we originally designed: a house on a frostwall foundation with no basement. It had some drawings that were very helpful for thinking about how to detail the foundation, and when Marc and Andrea and I were brainstorming about how to build the actual foundation, that detail was very helpful in figuring out what to do (although Marc might argue that it led to me being obsessed with details that weren't all that important).

What the book does not cover at all, however, is how to do a floor when your house is on a pier foundation. Both Marc and Peter were a bit concerned about how that was going to work, but it went pretty well in the PHPP model. Normally in a slab foundation, you'd lay down a really thick layer of expanded polystyrene foam insulation (EPS). This would isolate the interior of the house from the ground. Typically the ground under the house will be warmer than ambient, though, so the EPS doesn't have to do as much work as our floor has to do to keep the house warm.

So we are going with a fairly thick floor—11 7/8" thick, with 4" of polyisocyanurate rigid foam insulation. The floor joists will be I-joists, to minimize thermal bridging. The insulation between the floor joists will be dense-packed cellulose. One really nice thing about this is that the floor will have a lot less foam in it than a typical floor—only 4", rather than the typical 8" or more of styrofoam insulation below the slab that you'd see in a Passivhaus.

An additional complication is that normally to get a good air barrier on the slab of a Passivhaus, you'd have a polyethylene membrane under the slab. This would then connect to the wall air barrier with some kind of sticky tape or expanding foam tape. We don't have that option with the floor box, because there's no place to put the polyethylene membrane.

Instead, the bottom of the box will be sheathed with zip sheathing. Zip sheathing provides an excellent air barrier. The edges of each piece of zip sheathing will be taped together. Remember, this tape is on the bottom of the sheathing. The bottom of the sheathing will be resting on the LVL beam or on the pressure-treated sill plate. This means that the sheathing has to be taped before it's nailed to the plate or to the beam.

In order to accomplish this, Eli's team is going to build the floor box in sections, upside down. They are going to tape the seams on the bottom of each section before flipping that section. When the time comes to install the sections, they will (handwaving, Eli, help!) to seal the joins between the sections.

The joint between the floor-bottom sheathing and the outer wall sheathing will be sealed with a gasket or caulk, as shown below. I'm not sure what sort of gasket to use if we go that route. We'd talked about using iso-bloco tape to seal the edge, but that stuff is very expensive. Another option would be to use EPDM gaskets. I don't know how much the EPDM gaskets cost—maybe they're just as expensive—but I suspect they are cheaper. It may also be that caulk is a good option, although I've heard arguments to the contrary.

Our building site was relatively quiet last week. Concrete is curing, and our electrician set up the main panel and meter in anticipation of CVPS turning on the electricity this week. Ted and I also spoke with several solar installers to see about getting some PV panels at the roof ridge and also a solar hot water system. More on that as it unfolds.

The biggest news is that we recently partnered with Efficiency Vermont to pursue Passivhaus certification [follow the link to read their "About Us" page]. The cool part is that our house will be part of a research project to evaluate the suitability of Passivhaus construction for Vermont. They'll install monitoring equipment in our house and closely study its performance.

Peter Schneider, Efficiency Vermont's Passivhaus consultant, was particularly interested in studying our house because it has several unusual features: a pier foundation and partial shading. Vermont's abundance of sloping, ledgy lots makes pier foundation a tempting solution, and of course trees are rampant hereabouts. So hopefully we'll provide useful data for would-be Passivhausers in North America.

Peter was on vacation last week, so he hasn't gotten farther than the first few rounds of PHPP tweaking, but Marc helped pick up the slack. This will all probably change this week, and I'm probably jinxing things just by typing this, but so far it looks like we can pull off Passivhaus performance with the following general specs:

Schuco SI-82+ windows, which we ordered this week from European Architectural Supply in Lincoln, MA. The windows are PH-certified and made from uPVC. Yes yes, PVC is evil, but this is unplasticized PVC which is apparently a bit less evil. It's made without phthalates and can be recycled, at least in Europe. But hopefully the windows won't need recycling for a long long time.

Climatop Max and Climatop Ultra-N glass. The glass offered by Schuco is pretty darn impressive. For the south windows we upgraded to Climatop Max, which has a SHGC of 0.6, but for the rest of the house we went with the Climatop Ultra-N, which has an SHGC of 0.5. All the glass has a Ug of 0.105 (which PHPP callously rounds up to 0.11).

We haven't decided for sure on the HRV yet, but we'll probably either do the Zehnder ComfoAir 350 or the Paul by Zehnder Novus 300. The latter adds about $1,400 to the already formidable cost, but the efficiency is 93% as opposed to the ComfoAir's 84%, which would win us quite a bit within PHPP. Another knob to turn would be to add more polyiso under the floor or use larger I-joists — we'll hopefully do the cost-benefit analysis this week and reach a verdict.

It seems like the biggest advantage in our design is the ludicrously simple house shape. We're basically building a shoebox with a shed roof, which means there aren't many corners or thermal bridges undermining our envelope. Marc, Ben, and Eli already minimized thermal bridging before we decided to go for Passivhaus certification, so we're picking up a lot of PHPP points without having to change our plans.

We're waiting on a few more details, though, including some THERM data Peter is confirming with PHIUS. Hopefully that won't kick us back out of the ballpark, but as I said we still have some knobs left to turn.

After much waffling, Ted and I decided to bite the bullet and go for Passivhaus certification. More on this soon, but in the meantime we're busily weighing envelope upgrades in search of the best (read: cheapest) path to 4.75 kBTU/(f2year).

I am superstitious and have no immediate plans to change the website name. The web address, of course, will not change. I am now grateful that I was unable to get http://www.almostpassive.com, which is boldly carrying out its mission to tell visitors "All You Need To Earn Almost Passive Income Online! "

In other news I made a new house rendering, but don't get too attached because I'm likely to change it again in a few days. The new rendering places the solar panels on the top awning, which will be almost entirely unshaded and can fit 4.76 kW of DC goodness.

Things that will change in the next rendering are:

The colors, which are currently quite ugly. We may also change the ratio of cedar shingle and reverse board and batten siding.

The terrain and foundation, since our building site is a wooded rocky hillside and not an eerie CAD plain.

Awnings will appear over the lower windows, though they might be removable in some way to allow full winter heat gains.

A solar collector for hot water might appear in the gap between the two groups of windows, assuming we can afford it.

Things are going swimmingly. They poured the piers yesterday, and the dismal unending rain means that the concrete will probably cure to maximum strength. We've also hammered out all sorts of details with Eli, Marc, and Ben, and a lot of problems and challenges seem to be melting away. I figured out a more attractive and efficient way to add solar electric and hot water (new drawings forthcoming), and everything is looking great.

The problem? Our many envelope tweaks and revisions have brought us tantalizingly close to Passivhaus performance levels. This is bad simply because we now have to decide whether to bite the bullet and pay the extra $3,000 (or more) to make it happen.

The return on investment will be lousy. The difference in utility costs between the house we're building and a certified Passivhaus will probably be less than $40/year. We've already ordered Passivhaus-certified windows and are seriously considering paying extra for a Passivhaus-certified HRV because of its many good features. And of course we've minimized thermal bridging in the envelope and are wrapping the house in heaps and heaps of dense-pack cellulose, plus a 4" exoskeleton of polyisocyanurate.

But we haven't paid the $1,500 (give or take) to have Marc model the house in PHPP, the über-spreadsheet that analyzes every detail of a house's energy performance. And you can't have a certified Passivhaus without running it through PHPP.

Of course, PHPP is only the first step. Odds are that we'll fall a little shy of the Passivhaus performance requirements, so our next move will be to add more insulation somewhere or tighten up the envelope a bit better. And that will cost more money than it will ever save us.

So why bother? I don't think I'm attached to having a certified Passivhaus. As you can see from the name of the website, I'm quite satisfied with our not-quite-passiv status. Also, I seriously doubt it will make a difference on resale whether the house is certified or not, since it's going to be a freaky-efficient house either way.

But there's a symbolic value to getting certified. Only a handful of certified Passive Houses have been built in the United States, which means it's still an inspiring new concept. One of our major goals in building an energy-efficient house is to inspire other builders and homeowners, and having the Passivhaus label and certificate will help get the word out.

Several people working on the house, both directly and peripherally, have told me how exciting it is to work on such an efficient design. They really like seeing someone do it right and not cut corners on energy performance. We're trying to practice "trickle-down green building," meaning that the way we're building this house will hopefully catch on and eventually become the new normal, at least to some degree. Building an actual Passive House rather than an Almost Passive House might further this goal.

Does that count, or should we just return to the ROI and call it a day? I should add that building an affordable house is another one of our goals, because it's not very inspiring if only people with endlessly deep pockets can build this way.

I was recently asked a simple but excellent question: What makes it passive?

The word "passive" turns up a lot in green building, and it can refer to several different things. When I say we're building an almost passive house, I'm referring to the Passivhaus building approach that was standardized in Europe and inspired by energy-efficient building methods pioneered in North America. The Passive House Institute US site summarizes:

A "passive" house achieves overall energy savings of 60-70% and 90% of space heating without applying expensive "active" technologies like photovoltaics or solar thermal hot water systems. Energy losses are minimized, and gains are maximized. Superinsulation and air-tight construction minimize losses.

Passivhaus certification is somewhat easier to attain in Europe than in North America, mostly because of their relatively moderate climate, but also because you can buy much whizzier building products over there (see my post on European windows).

After a considerable amount of waffling, Ted and I decided not to go for full Passivhaus certification, but we're still planning to use as many passive house techniques as we can (superinsulation, avoiding thermal bridges, sealing the house extremely tightly, using mechanical fresh-air systems, etc.).

In passive solar building design, windows, walls, and floors are made to collect, store, and distribute solar energy in the form of heat in the winter (Passive Solar Heating) and reject solar heat in the summer (Passive Solar Cooling). This is called "passive" solar design (or climatic design) because, unlike "active" ( solar heating, photovoltaic, etc.) solar systems, passive solar systems do not involve the use of mechanical or electrical devices, fans, pumps, etc.

Passive solar home design was undoubtedly discovered by cave dwellers who noticed that south-facing caves were more comfortable year-round than caves facing other directions (cave dwellers in the southern hemisphere would have chosen north-facing caves). This is because the sun is angled low in winter and high in summer, meaning that winter light and heat will penetrate deeply into a south-facing cave, and summer sunlight will be blocked by the cave overhang. Furthermore, a cave with a solid earth floor retains winter heat gains even after sunset, because earth floors have a high thermal mass which absorbs heat during the day and then slowly releases it at night.

The cliff-dwellings at Mesa Verde in southwest Colorado are the textbook example of passive solar building. The dwellings face south and are protected from the hot summer sun by a gigantic overhang, but during the winter they are bathed in light.

The advent of mechanical heating and cooling systems made it easier for builders to ignore passive solar techniques. The problem got worse when people started building houses with ginormous windows, often facing a nice view in a direction other than south. Ted's parents' house has a great room with floor-to-ceiling windows facing a lovely view toward the west. Every afternoon the room is flooded with light, which brings welcome solar gains in winter (they can turn off their heater for much of the day) but way too much heat during the summer.

It is much easier to achieve Passivhaus certification if you maximize solar gains with clever window placement, thereby reducing the need for mechanical heating. Our building site isn't perfect for passive solar since we have quite a few trees blocking the sun toward the south, but it's not too bad, particularly since most of those trees will lose their leaves every autumn.

To get the maximum bang for our passive solar buck, I used SketchUp to simulate the solar shading at different times of year. I entered our latitude and longitude, and then I told SketchUp to show me what shadows will form on different dates (including the date of this blog post). Our house has big windows facing south, so they'll be our primary source for solar gain, and I tweaked the length of the roof overhang so it will admit plenty of sun in winter without allowing too much unwanted summer heat:

We're being careful to order windows with a high solar heat gain coefficient (SHCG), which means that the glass won't filter out too much of the warm sunlight. Again, refer to my future post on windows for more about SHGC.

Random Bits

Ordinary houses breathe through leaky joints and poor seals, losing heat and wasting energy. But our house won't leak, so we'll use a heat recovery ventilator (HRV) to admit fresh air and expel stale air, transferring heat from one stream to the other.